Imagine a planet so incredibly hot that its entire surface is essentially a global magma ocean, yet it still manages to hold onto a thick atmosphere—sounds almost unbelievable, doesn’t it? But here's where it gets controversial: recent observations suggest this might actually be happening on a distant world. In a groundbreaking study led by Carnegie Institution scientists using NASA's James Webb Space Telescope (JWST), the strongest evidence yet has been found for an atmosphere enveloping a rocky exoplanet named TOI-561 b, which orbits so close to its star that its surface is almost certainly a molten sea of lava. The findings, published in The Astrophysical Journal Letters, reveal a surprisingly dense gaseous layer hovering above this planet’s fiery surface, defying long-held expectations about the fragility of atmospheres on ultra-hot rocky worlds.
TOI-561 b isn’t your average planet. It weighs about twice as much as Earth but circles its star at a distance roughly 40 times closer than Mercury’s orbit around the Sun. Its year lasts just over 10 hours, and one hemisphere is permanently baked in daylight due to tidal locking. Even though the star it orbits is slightly cooler and less massive than our Sun, the planet's extreme proximity exposes it to intense radiation, heating its surface to unimaginable temperatures.
Nicole Wallack, a Postdoctoral Fellow at Carnegie, highlights the surprise: standard planetary theories predict that such a small and hot planet should lose its atmosphere relatively quickly after formation—yet, this one still retains a thick gaseous envelope. Interestingly, unlike many planets in our Solar System that shed their primordial atmospheres long ago, TOI-561 b’s atmosphere appears to have persisted for billions of years, despite its host star being significantly older than the Sun.
This persistent atmosphere helps explain the planet’s surprisingly low density. Johanna Teske, an astronomer involved in the study, notes that TOI-561 b’s bulk density is lower than what you'd expect if it were just a mixture of rock and metal, but it isn’t classified as a super-puff planet—a type known for exceptionally low densities. She elaborates that the planet's composition may involve a core made of lighter materials combined with a mantle that has lower-density rocks.
A fascinating aspect of TOI-561 b is its formation history. Since it orbits an iron-poor star that is around twice as old as the Sun, it likely formed in a very different chemical environment, hinting at the possibility that rocky planets in the early universe may underlie different structural compositions compared to those forming today in our neighborhood. This raises provocative questions: Did the universe's earlier epochs produce rocky planets with distinct characteristics? Could this be a whole new class of ancient worlds?
However, mere composition cannot fully explain the planet’s properties. The researchers turned their focus to its atmosphere, hypothesizing that a significant gaseous layer could make the planet appear larger and thus less dense in measurements. To confirm this, they used JWST’s Near-Infrared Spectrograph to observe how the planet's temperature responded when it moved behind its star—a method similar to studies conducted on the TRAPPIST-1 system.
By analyzing the drop in brightness before, during, and after the planet’s secondary eclipse, the team estimated the temperature on TOI-561 b’s dayside. If it were only a bare rocky surface, temperatures could soar up to approximately 4,900°F (2,700°C). Surprisingly, the data indicate a temperature closer to 3,200°F (about 1,800°C)—still scorching, but significantly cooler than the expected temperature for a rock-only surface.
Could this cooling be attributed solely to internal heat transfer? The scientists considered that a magma ocean might move some heat to the planet’s nightside, but without an atmosphere, the dark side would likely cool and solidify rapidly. Other possibilities, like a thin vapor layer over the lava, just don’t seem sufficient to explain the cooler dayside temperatures observed.
Instead, experts like co-author Anjali Piette argue that only a thick, volatile-rich atmosphere can produce such effects. Winds within this atmosphere could transport heat globally, while molecules such as water vapor absorb infrared radiation, making the planet appear cooler from afar. Additionally, reflective clouds of silicate minerals might further diminish the observed temperature by reflecting starlight.
Despite JWST’s compelling evidence for this atmospheric presence, a fundamental question remains: how can a small, intensely irradiated planet keep such a thick gaseous layer? Gases tend to escape quickly into space under these conditions, yet TOI-561 b’s atmosphere endures, hinting at complex processes at play. Co-author Tim Lichtenberg suggests a dynamic equilibrium: gases outgas from the molten interior while the magma itself reabsorbs some volatiles, hinting at a planet much richer in volatile elements than Earth—more akin to a “wet lava ball,” as he puts it.
Johanna Teske reflects on the broader implications: this discovery challenges existing models and raises fresh questions about atmospheric retention and magma-atmosphere interactions on planets exposed to extreme radiation. As the first results from JWST’s extensive observational campaign on the TOI-561 system—more than 37 hours of continuous monitoring—the team now aims to produce detailed temperature maps and better understand the atmospheric makeup.
And this is the part most people miss: what does this mean for our understanding of planetary formation and habitability? Could ancient, rocky worlds like TOI-561 b harbor more complex atmospheres than previously thought, even billions of years after their formation? Or are we on the verge of rewriting what we believe about planetary survival in extreme conditions? We invite you to ponder these questions and share your thoughts in the comments—do you agree that this discovery might reshape how we see the universe’s most extreme planets or do you think there’s more to uncover before drawing conclusions?
